Variation in host preferences of malaria mosquitoes is mediated by skin bacterial volatiles.

The host preferences of the anthropophilic mosquito species in the Anopheles gambiae complex (Diptera: Culicidae) are mediated by skin bacterial volatiles. However, it is not known whether these mosquitoes respond differentially to skin bacterial volatiles from non-human host species. In this study, the responses of two malaria mosquito species in the An. gambiae complex, Anopheles gambiae s.s. (hereafter, An. gambiae) and Anopheles arabiensis, with different host preferences, to volatiles released from skin bacteria were tested. Skin bacteria collected from human, cow and chicken skin significantly increased trap catches; traps containing bacteria collected from human skin caught the highest proportions of An. gambiae and An. arabiensis. Traps with bacteria of human origin caught a significantly higher proportion of An. gambiae than of An. arabiensis, whereas bacterial volatiles from the chicken attracted significantly higher numbers of An. arabiensis than of An. gambiae. Additionally, An. gambiae showed a specialized response to volatiles from four specific bacteria, whereas An. arabiensis responded equally to all species of bacteria tested. Skin bacterial volatiles may therefore play important roles in guiding mosquitoes with different host preferences. The identification of these bacterial volatiles can contribute to the development of an odour blend that attracts mosquitoes with different host preferences.

Variation in malaria transmission dynamics in three different sites in Western kenya.

The main objective was to investigate malaria transmission dynamics in three different sites, two highland villages (Fort Ternan and Lunyerere) and a lowland peri-urban area (Nyalenda) of Kisumu city. Adult mosquitoes were collected using PSC and CDC light trap while malaria parasite incidence data was collected from a cohort of children on monthly basis. Rainfall, humidity and temperature data were collected by automated weather stations. Negative binomial and Poisson generalized additive models were used to examine the risk of being infected, as well as the association with the weather variables. Anopheles gambiae s.s. was most abundant in Lunyerere, An. arabiensis in Nyalenda and An. funestus in Fort Ternan. The CDC light traps caught a higher proportion of mosquitoes (52.3%) than PSC (47.7%), although not significantly different (P = 0.689). The EIR's were 0, 61.79 and 6.91 bites/person/year for Fort Ternan, Lunyerere and Nyalenda. Site, month and core body temperature were all associated with the risk of having malaria parasites (P < 0.0001). Rainfall was found to be significantly associated with the occurrence of P. falciparum malaria parasites, but not relative humidity and air temperature. The presence of malaria parasite-infected children in all the study sites provides evidence of local malaria transmission.

An exploratory survey of malaria prevalence and people's knowledge, attitudes and practices of mosquito larval source management for malaria control in western Kenya.

A large proportion of mosquito larval habitats in urban and rural communities in sub-Saharan Africa are man-made. Therefore, community-based larval source management (LSM) could make a significant contribution to malaria control in an integrated vector management approach. Here we implemented an exploratory study to assess malaria prevalence and people's knowledge, attitudes and practices on malaria transmission, its control and the importance of man-made aquatic habitats for the development of disease vectors in one peri-urban lowland and two rural highland communities in western Kenya. We implemented monthly cross-sectional malaria surveys and administered a semi-structured questionnaire in 90 households, i.e. 30 households in each locality. Malaria prevalence was moderate (3.2-6.5%) in all sites. Nevertheless, residents perceived malaria as their major health risk. Thirty-two percent (29/90) of all respondents did not know that mosquitoes are responsible for the transmission of malaria. Over two-thirds (69/90) of the respondents said that mosquito breeding site could be found close to their homes but correct knowledge of habitat characteristics was poor. Over one-third (26/67) believed that immature mosquitoes develop in vegetation. Man-made pools, drainage channels and burrow pits were rarely mentioned. After explaining where mosquito larvae develop, 56% (50/90) felt that these sites were important for their livelihood. Peri-urban residents knew more about mosquitoes' role in malaria transmission, could more frequently describe the larval stages and their breeding habitats, and were more likely to use bed nets even though malaria prevalence was only half of what was found in the rural highland sites (p<0.05). This was independent of their education level or socio-economic status. Hence rural communities are more vulnerable to malaria infection, thus calling for additional methods to complement personal protection measures for vector control. Larval source management was the most frequently mentioned (30%) tool for malaria control but was only practiced by 2 out of 90 respondents. Targeting the larval stages of malaria vectors is an underutilized malaria prevention measure. Sustainable elimination or rendering of such habitats unsuitable for larval development needs horizontally organized, community-based programs that take people's needs into account. Innovative, community-based training programs need to be developed to increase people's awareness of man-made vector breeding sites and acceptable control methods need to be designed in collaboration with the communities.

A simple method for sampling indoor-resting malaria mosquitoes Anopheles gambiae and Anopheles funestus (Diptera: Culicidae) in Africa.

Sampling indoor resting African malaria vectors is traditionally done by hand catches with oral or mechanical aspirators and pyrethrum spray catches (PSCs). In this study, we designed and briefly evaluated an inexpensive but practical alternative by using a cloth resting box or wicker resting basket and a ceiling net. Evaluations were performed in greenhouse and field situations in rural Kenya by comparing capture rates of Anopheles gambiae s.l. and Anopheles funestus (Giles) in these traps to hand collections and PSCs. A resting box and a ceiling net when used together collected more mosquitoes than a single collector using a hand-held aspirator but only one-third the number collected by PSCs. At sites where PSCs are impractical, a resting box and ceiling net can be effectively used as an alternative to hand catches in malaria surveillance.

Extent of digestion affects the success of amplifying human DNA from blood meals of Anopheles gambiae (Diptera: Culicidae).

The success of distinguishing blood meal sources of Anopheles gambiae Giles through deoxyribonucleic acid (DNA) profiling was investigated by polymerase chain reaction (PCR) amplification at the TC-11 and VWA human short tandem repeats (STR) loci. Blood meal size and locus had no significant effect on the success of amplifying human DNA from blood meals digested for 0, 8, 16, 24 and 32 h (P = 0.85 and 0.26 respectively). However, logistic regression found a significant negative relationship between time since ingestion and the success probability of obtaining positive PCR products among meals digested for between 8 and 32 h (P = 0.001). Approximately 80% of fresh blood meals were successfully profiled. After 8 h, the proportion of blood meals that could be successfully profiled decreased slowly with time after ingestion, dropping to below 50% after approximately 15 h. There was no significant difference in the success of amplifying human DNA from blood meals of mosquitoes killed at time 0 and 8 h after ingestion (P = 0.272).

Analysis of genetic variability in Anopheles arabiensis and Anopheles gambiae using microsatellite loci.

We analysed genetic variability in Anopheles arabiensis and Anopheles gambiae populations using microsatellite loci to determine whether the Rift Valley restricts the flow of genes. Deviations from Hardy-Weinberg expectations were significant, and were most likely to be due to the high frequency of null alleles observed. An. arabiensis populations occurring between 40 and 700 km apart across the Eastern arm of the Rift Valley were not differentiated (pair-wise F(ST) range: 0.0033-0.0265, P > 0.05). Neither were An. gambiae populations from Asembo Bay and Ghana (F(ST): 0.0063, P > 0.05) despite a geographical separation of about 5000 km. In contrast, significant differentiation was observed between An. gambiae populations from Asembo Bay and Kilifi (about 700 km apart; F(ST) = 0.1249, P < 0.01), suggesting the presence of a barrier to gene flow.